Why Weight Matters Before, During, and After Pregnancy

By Elizabeth M. Ward, M.S., R.D.

"How much weight should I gain?" is a common question for expectant moms. Your weight before – and after – pregnancy matters, too, because it can influence your health and your baby's well-being.

Before Pregnancy

Your body weight before pregnancy may affect your chances of becoming pregnant and the risk of pregnancy complications. Being underweight or overweight can lengthen the time it takes to conceive. Starting pregnancy at a healthy weight provides baby with better odds of developing normally, and minimizes problems such as high blood pressure and gestational diabetes in mom.

Here's what BMI means. If it's less than 18.5, you're underweight. A BMI of 18.5 to 24.9 is considered healthy. A BMI ≥ 25 indicates overweight, and obesity is defined as a BMI ≥ 30.

During Pregnancy: What's the right weight gain for you?

The Institute of Medicine (IOM) and the National Research Council (NRC) have joined together to provide research-based guidelines for pregnancy weight gain. Pre-pregnancy BMI and how many children you're carrying serve as the guideposts for the number of pounds to put on when you're expecting.

BMI is the starting point for pregnancy weight gain recommendations. Women with a higher BMI are often advised to gain less weight; those with a lower BMI may need to put on more pounds with pregnancy.

Here is a summary of weight gain guidelines for a single baby:

Your BMI Is

Your Total Weight Gain Should Be

Less than 18.5

28 to 40 pounds

18.5 to 24.9

25 to 35 pounds

25 to 29.9

15 to 25 pounds

30 or greater

11-20 pounds

For twins:

Your BMI Is

Your Total Weight Gain Should Be

Less than 18.5

Ask your doctor

18.5 to 24.9

37 to 54 pounds

25 to 29.9

31 to 50 pounds

30 or greater

25 to 42 pounds

Note that the IOM/NRC guidelines are for total weight gain in the 40 or so weeks of pregnancy. Weight gain begins in earnest at the start of the second trimester, which is when you'll require more calories, about 340 a day in the second trimester and about 450 more than your pre-pregnancy diet during the third trimester, to support your baby's growth.

What if you don't gain the right number of pregnancy pounds? Being off by just a few pounds is generally not a cause for concern. Always ask your doctor or nurse-midwife about the weight gain that's right for you.

Losing the Baby Weight

You may be yearning to fit into your pre-pregnancy clothes, but hold off on drastic calorie reductions (which are never a good idea, anyway). Chances are, you're battling exhaustion. A healthy diet with enough calories for slow and steady weight loss provides the energy to cope with a needy infant, and with your other duties.

It takes a while to recover from giving birth and to get your body back to normal, especially when you're sleep-deprived. While there is no rush to shed the baby weight, you should make an effort to take off the extra pounds within the first year of giving birth. Research suggests it's harder to take off the weight after 12 months have passed.

And, achieving and maintaining a healthy BMI is important for your health and well being, particularly if you should conceive again.

About the Author

Elizabeth M. Ward, M.S., R.D., is a registered dietitian, a writer, and mother of three. She has worked at the Joslin Diabetes Center and the American Heart Association, and for seven years counseled children and adults about healthy eating and disease prevention at Harvard Vanguard Medical Associates in Boston.